1. Field of the Invention
This invention pertains to repeatable release holdback bars and more particularly to mechanically actuated hydraulic holdback bars.
2. Description of the Prior Art
Aircraft operating from aircraft carrier decks or other short take-off surfaces usually require catapult assistance in order to reach minimum flying speed. A holdback bar is required to restrain the aircraft against increasing catapult forces and engine thrust prior to take-off. The bar must restrain the aircraft until a pre-determined combined force is present and then release the aircraft cleanly and rapidly to enable smooth acceleration to flying speed.
Prior holdback bars have included, for example, tensile failure links which part upon reaching the predetermined force, and which must be replaced for each launch. Expended tensile links create a clutter hazard to personnel and may be ingested through jet engine intake ducts on aircraft awaiting take-off. Other holdback bars are typified by U.S. Pat. No. 3,813,065 to Harold W. Hallesy et al. dated May 28, 1974, and U.S. Pat. No. 3,578,273 to Thomas Mulgrave, dated May 11, 1971.
The Mulgrave patent discloses a hydraulic holdback bar which compresses hydraulic fluid within a pressure chamber. Upon reaching a pre-determined pressure, the fluid moves a spring biased ball off its seat and flows into a receiving chamber. This transfer of fluid permits relative movement between a plunger and housing triggering release of the aircraft. The spring loaded ball release valve operates as a pressure regulator and only permits fluid to flow from the pressure chamber at a pre-determined pressure. The Hallesy et al. patent discloses a holdback bar which utilizes a one piece split, chamfered locking ring as a locking member between other load carrying members. Application of pre-determined loads to the load carrying members releases the locking ring from its locking condition by flexure of the ring. Mechanical devices generally are subject to changes in calibrated release force caused by changes in frictional resistance of the various mechanical parts. A mechanical holdback bar therefore is difficult to calibrate and may not retain its calibrated release point after a few uses.